1. Field of the Invention
This invention relates to the field of subsurface exploration and, more particularly, to logging techniques for detecting and locating fractures in earth formations.
2. Description of Related Art
Electromagnetic (EM) logging tools have been employed in the field of subsurface exploration for many years. These logging tools or instruments entail an elongated support equipped with antennas that are operable as sources or sensors. The antennas on these tools are generally formed as loops or coils of conductive wire. In operation, a transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (xe2x80x9cmudxe2x80x9d) and into the surrounding formations. The emitted energy interacts with the borehole and formation to produce signals that are detected and measured by one or more receiver antennas. The detected signals reflect the interaction with the mud and the formation. By processing the detected signal data, a log or profile of the formation and/or borehole properties is determined.
Conventional EM logging techniques include xe2x80x9cwirelinexe2x80x9d logging and logging-while-drilling (LWD) or measurement-while-drilling (MWD). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD/MWD entails attaching the instrument disposed in a drill collar to a drilling assembly while a borehole is being drilled through earth formations. A developing method, sometimes referred to as logging-while-tripping (LWT), involves sending a small diameter xe2x80x9crun-inxe2x80x9d tool through the drill pipe to measure the downhole properties as the drill string is extracted or tripped out of the hole.
A coil or loop-type antenna carrying a current can be represented as a magnetic dipole having a magnetic moment strength proportional to the product of the current and the area encompassed by the coil. The magnetic moment direction can be represented by a vector perpendicular to the plane of the coil. In the case of more complicated coils, which do not lie in a single plane (e.g. saddle coils as described in published U.S. patent application Ser. No. 20010004212 A1, published Jun. 21, 2001), the direction of the dipole moment is given by: rxc3x97dl and is perpendicular to the effective area of the coil. This integral relates to the standard definition of a magnetic dipole of a circuit. See J. A. Stratton, ELECTROMAGNETIC THEORY, McGraw Hill, N.Y., 1941, p. 235, FIG. 41. Integration is over the contour that defines the coil, r is the position vector and dl is the differential segment of the contour.
In conventional EM logging tools, the transmitter and receiver antennas are typically mounted with their axes along, or parallel, to the longitudinal axis of the tool. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). An emerging technique in the field of well logging is the use of tools with tilted or transverse antennas, i.e., where the antenna""s axis is not parallel to the support axis. These tools are thus implemented with antennas having a transverse or tilted magnetic dipole moment (TMD). One logging tool configuration comprises triaxial antennas, involving three coils with magnetic moments that are not co-planar. The aim of these TMD configurations is to provide EM measurements with directed sensitivity. Logging tools equipped with TMDs are described in U.S. Pat. Nos. 6,044,325, 4,319,191, 5,115,198, 5,508,616, 5,757,191, 5,781,436 and 6,147,496.
EM propagation tools measure the resistivity (or conductivity) of the formation by transmitting radio frequency signals into the formation and using spaced-apart receivers to measure the relative amplitude and phase of the detected EM signals. These tools transmit the EM energy at a frequency in the range of about 0.1 to 10 MHz. A propagation tool typically has two or more receivers disposed at different distances from the transmitter(s). The signals reaching the receivers travel different distances and are attenuated to different extents and are phase-shifted to different extents. In analysis, the detected signals are processed to derive a magnitude ratio (attenuation) and phase difference (phase shift). The attenuation and phase shift of the signals are indicative of the conductivity of the formation. U.S. Pat. Nos. 4,899,112 and 4,968,940 describe conventional propagation tools and signal processing.
In addition to the formation resistivity, identification of subsurface fractures is important in hydrocarbon exploration and production. Fractures are cracks or breakages within the rocks or formations. Fractures can enhance permeability of rocks or earth formations by connecting pores in the formations. Fractures may be filled with formation fluids, either brine or hydrocarbons. If a fracture is filled with hydrocarbons, it will be less conductive, i.e., a resistive fracture. Wells drilled perpendicularly to resistive fractures tend to be more xe2x80x9cproductivexe2x80x9d (i.e., produce lager quantities of hydrocarbons). Thus, the determination of a resistive fracture""s orientation may help improve oil and gas production. In addition, the orientation of a fracture provides the direction of principal stress, which affects the stability of the well and it helps in predicting which well trajectory will be the most stable. Knowledge of fracture orientations also aids in the prediction of fracture strengths of the earth formation. Furthermore, the presence of fractures may indicate that the mud weight used for drilling the well is too high so as to cause fracture of the rock.
Methods and systems have been developed for detecting fractures and determining their orientation. For example, U.S. Pat. No. 3,668,619 describes the rotation of a logging tool having a single acoustic transducer that senses the reflected acoustic energy to detect fractures. U.S. Pat. No. 5,121,363 describes a method for locating a subsurface fracture based on an orbital vibrator equipped with two orthogonal motion sensors and an orientation detector. U.S. Pat. No. 4,802,144 uses the measurement of hydraulic impedance to determine fractures. U.S. Pat. No. 2,244,484 measures downhole impedance to locate fractures by determining propagation velocity.
There remains a need for improved techniques for detecting and locating fractures, and for determining their orientations, particularly using propagation-type tools.
The invention provides a method for locating a fracture in an earth formation using a propagation tool disposed in a borehole traversing the formation, the tool having a longitudinal axis. The method comprises transmitting electromagnetic energy from a transmitter antenna disposed on the propagation tool with its magnetic moment at an angle with respect to the longitudinal tool axis; measuring voltage signals detected at a plurality of receiver antennas disposed on the propagation tool with their axes at an angle with respect to the longitudinal tool axis and oriented in different directions from one another, the voltage signals being related to the transmitted electromagnetic energy; associating the measured voltage signals with a plurality of azimuthal angles; and shifting at least one of the measured voltage signals by a predetermined angle and processing the shifted and unshifted signals to locate the fracture.
The invention provides a system for locating a fracture in an earth formation. The system comprises a propagation tool having a longitudinal axis and adapted for disposal within a borehole traversing the formation; a transmitter antenna disposed on the tool with its magnetic moment at an angle with respect to the tool axis; a plurality of receiver antennas disposed on the tool with their axes at an angle with respect to the tool axis and oriented in different directions from one another, the antennas adapted to detect voltage signals associated with electromagnetic energy transmitted by the transmitter antenna; processing means to measure the voltage signals detected by said receiver antennas; processing means to associate the measured voltage signals with a plurality of azimuthal angles; and processing means to shift at least one of the measured voltage signals by a predetermined angle and to process the shifted and unshifted signals to locate the fracture.
The invention provides a method for locating a fracture in an earth formation penetrated by a borehole. The method comprises moving a propagation tool in the borehole, the tool having a longitudinal axis and including a first transmitter antenna disposed thereon with its magnetic moment at a right angle to the tool axis and a plurality of receiver antennas disposed thereon with their axes at right angles to the tool axis; transmitting electromagnetic energy using the first transmitter antenna; measuring voltage signals detected at the plurality of receiver antennas, the signals being related to the transmitted electromagnetic energy; associating the measured signals with a plurality of azimuthal angles; shifting at least one of the measured signals by a predetermined angle; and locating the fracture using the shifted and unshifted signals.
The invention provides a method for locating a fracture in an earth formation using a logging tool disposed in a borehole traversing the formation, the tool having a longitudinal axis. The method comprises transmitting electromagnetic energy from a transmitter antenna disposed on the tool with its magnetic moment at an angle with respect to the longitudinal tool axis; measuring voltage signals detected with a receiver antenna disposed on the tool with its axis at an angle with respect to the longitudinal tool axis, the voltage signals being related to the transmitted electromagnetic energy; determining a second harmonic associated with the measured voltage signals; and performing a calculation on the second harmonic to locate the fracture.
The invention provides a system for locating a fracture in an earth formation. The system comprises a logging tool having a longitudinal axis and adapted for disposal within a borehole traversing the formation; a transmitter antenna disposed on the tool with its magnetic moment at an angle with respect to the tool axis; a receiver antenna disposed on the tool with its axis at an angle with respect to the tool axis, the antenna adapted to detect voltage signals associated with electromagnetic energy transmitted by the transmitter antenna; processing means to determine a second harmonic associated with voltage signals detected with the receiver antenna; and processing means to perform a calculation on the second harmonic to locate the fracture.